leetcode-142-环形链表②

题目描述：

 

方法一：O(n) O(n)

# Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution(object):
    def detectCycle(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """
        visited = set()
        node = head
        while node:
            if node in visited:
                #return visited.index(head.val)
                return node
            else:
                visited.add(node)
                node = node.next
        return None

方法二：快慢指针 O(N) O(1)

class Solution(object):
    def detectCycle(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """
        fast, slow = head, head 
        while True: 
            if not (fast and fast.next): 
                return 
            fast, slow = fast.next.next, slow.next 
            if fast == slow: 
                break 
        fast = head 
        while fast != slow: 
            fast, slow = fast.next, slow.next 
        return fast